Amorphous Silicon Photodiodes for Replacing Degenerated Photoreceptors in the Human Eye

Abstract

An ambitious neurotechnology program has been started in Germany in 1995, in the long run aiming at the realization of visual prostheses for blind people. A broad technological approach has been chosen which besides crystalline silicon microelectronics also involves amorphous silicon photodiodes for subretinal implantation, because thin film technology may be able to offer better solutions to some of the complex problems. Special topics we report on here include the development of low temperature deposition techniques for enabling the use of flexible plastic or bio-degradable substrate foils, the study of protective and bio-compatible coatings, as well as novel contacting and energy supply schemes. The key issue for stimulating retina cells by the use of technically generated photocurrents is an optimum capacitive coupling to these cells. For this purpose we study several contact layers (p-doped a-Si:H, microcrystalline Si, metal-induced crystallization) which provide high perpendicular but at the same time low lateral conductivity, thereby greatly reducing parasitic losses to the surrounding tissue. The photovoltaic mode of operation of the implanted photodiodes may be limited due to shortcomings in the charge transfer to the nerve cells, in which case additional infrared energy has to be coupled into the devices. Local light-induced stimulation can then be realised by using an a-Si:H i-layer as a photoresistor on top of the IR-sensitive solar cell.

Design and characterisation of graphene‐based nano‐photodiode array device for photo‐stimulation of subretinal implant

“…Over the past two decades, several research groups have reported many subretinal implant design approaches. Contemporary approaches for the subretinal implant are based on silicon micro photodiode array and metallic electrodes [16][17][18][19][20][21][22][23]. However, this approach is typified by relatively low spatial resolution, high rigidity, complication in a medical procedure, the advancement of corrosion in the implants after few months and damages to retinal tissues by stiff Si substrates.…”

Amorphous silicon transistors on ultrathin steel foil substrates

“…Steel 1,2 and plastic 3-10 foils have been explored as substrates for thin-film transistors ͑TFTs͒ and for thin-film pixels that combine an organic light-emitting diode with an amorphous silicon (a-Si:H) TFT. 11 Investigations of the mechanical bending of TFT/foil substrate structures show that TFTs 12 and photodiodes 13 are surprisingly rugged when made on thin foils. This observation agrees with the results of a theoretical analysis of the bending of thin-film/foil substrate couples, which also shows that stresses on the film can be reduced to virtually zero if the proper combination of materials is used.…”